Indicators are substances that change color within a certain pH range.
Used in titrations to determine the end point.
Comprise a weak acid (
HD) and its conjugate base (IND
-).
Color change depends on which form (acid or base) is predominant.
Fundamental Mechanics of Indicators
Weak acid (HD) ionizes in water, producing H3O+ and its conjugate base (IND
-).
The acid and base forms have different colors; e.g., yellow and blue.
The color seen is based on the dominant form of the indicator in solution.
Equilibrium shift towards the acid side shows one color, while a shift towards the base side shows another.
Equilibrium Shift and Color Change
Adding an acid increases H3O+ concentration, shifting equilibrium towards the acid form, resulting in a color change towards the acid side color (e.g., yellow).
Adding a base decreases H3O+ concentration, shifting equilibrium towards the base form, causing a color change towards the base side color (e.g., red).
Transition Point
Occurs at a specific pH where there are equal parts acid and base form of the indicator.
At the transition point, a mixture of the two colors is observed, creating a secondary color (e.g., yellow and red make orange).
Practical Application
Indicators like Bromocresol Green change color based on their acidic (yellow) or basic (blue) form.
The transition point leads to a mix of these colors, producing a new color (e.g., green).
Indicators and pH
Indicators are selected based on their color change range relative to the pH being measured.
The pH at which an indicator changes color is linked to the indicator's Ka value.
Indicators provide visual cues for acidity or basicity of a solution based on their color changes at specific pH levels.
Conclusion
Understanding the relationship between indicator color, pH, and equilibrium shifts is critical for correctly interpreting titration outcomes.
Tomorrow's class will further explore indicators, including how to calculate their Ka values.